Larry W4CSC wrote:
Jeff wrote in :


I'm confused. Perhaps this is why I left hardware engineering for
software. I thought the purpose of a diode was to pass current one
way, and block it the other. Did this fail because the passed current
was too high, or because the blocked voltage was too high?


When he hooked it up backwards, the diode was forward biased straight
across the battery's terminals. Current went through the roof, welding the
junction to a short which, microseconds later, blew the fuse. Once welded
to a short, unless you can really melt the mounting, it stays a
short....across the coil, not in series with it. It's a short both ways at
that point.

Thanks. The aspect that eluded me was that the dead short to ground
would pass "infinite" current until something popped, like the
breakers. Most circuits with diodes, such as rectifiers, have some
resistance implied elsewhere.




This diode's function is to short the relay's coil when the coil's field
collapses, producing inductive kick which forward biases the diode for a
few milliseconds each time power is removed from the coil. Otherwise, the
inductive kick, which could be several hundred volts for a few microseconds
like the spark from an ignition coil, could destroy the control electronics
or switch contacts which arc each time it is de-energized. With the diode
in place, inductive kick produces .6v pulses, the forward bias voltage of
the diode as it sinks the kick's power current pulse.

Yup. Subtle things like that I tend to remember. Its only the
obvious that I forget.

....
Why can't everything just be digital?


It is digital...(c; 1 = relay on.....0 = relay off...but it's not
hexidecimal..making it harder for a software man to comprehend...hee hee.

I beg your pardon. I'm software from the "old school," as in front
panel lights and flipping switches to install bootstraps and debug. I
still have the front panel from the first computer I owned - a Data
General Nova 1200.
http://ed-thelen.org/comp-hist/dg-nova.html

And since I came from the "mini" world, I learned Octal long before Hex.



While I'm on the subject, since I have twin engines, some of my
instruments, such as the fuel gauge are fed by the key switch from one
engine. Is there a way to hook up 2 diodes to allow either feed to
work/ Radio Shack part numbers please ;-)



Any 1 amp, 50V piv or higher rectifier will work.....for this OR
circuit...(c;

Hook the anodes of the diodes, one to each power switch on each engine.
Hook the cathodes (banded end) together to the common power + lead to each
fuel guage. If either engine is turned on, its diode will conduct turning
on both fuel guages. (classic OR gate - DDL logic!)

engine 1----------------||---------\
\
\
engine 2----------------||------------\-----------fuel guage +

Circuit may be too simple for mainframe engineers...(c;

Thanks. It looks too obvious for me.

Ken mentioned that the voltage drop could affect the gauges. I can
see how the cheap fuel gauge might have a problem but it made me
wonder if the water temp gauges would be affected. Not that I would
need the diodes for them, but the alternator output can vary from 12
something (turned off) to 14.5, depending on what's going on in the
regulator's little mind. I have to believe that such fluctuations
don't make much of a difference in the temp readout, but I couldn't
find a spec on the Teleflex site that indicated they were stable over
a reasonable voltage range. Any thoughts?